The CD4 molecule has been identified as the receptor for HIV envelope protein. Recently, the natural ligand for CD4 on antigen presenting cells have been tentatively localized to the N-terminal domain of the beta chain of MHC Class II. It was postulated that the MHC Class II and HIV bind the non-polymorphic CD4 via similar conserved regions. A hydrophilic septamer was identified displaying a high degree of homology between gp41 of HIV and the beta-1 domain of HLA-DR and - DQ. Both the HIV and MHC Class II derived sepatmers were synthesized. Incubation of these peptides, but not control peptides, with CD4 positive cells at 37 degrees C for 45 min resulted in reduced binding of anti-CD4 antibodies (OKT4, OKT4a and Leu3) to the cells. This peptide mediated reduction of binding to CD4 could be blocked in the presence of chloroquine. The binding of antibodies directed against other surface antigens, were unaffected by pre-incubation with the peptides. The temperature requirement and the sensitivity to chloroquine suggest that the peptides induced partial modulation of the CD4 molecules via receptor mediated endocytosis. In addition, flow cytometry showed that biotinylated chicken albumin conjugates of the peptides can bind directly to CD4 bearing CEM cells, but not to a CD4 negative CEM mutant or to B cell lines. This binding could be partially inhibited in the presence of mouse monoclonal anti-CD4 antibodies. Therefore, these findings suggest that the homologous regions of HIV and MHC Class II, which we have identified, may be the sites involved in binding of AIDS virus and MHC Class II antigens to CD4 on human T cells. Rabbit antiserum and murine mAb specific for the HIV derived peptide were found to recognize intact inactivated virions and also stain native human class II molecules on B cell lines and on murine L cell transfected with DR, DQ and DP.